The function of wide outside through lanes as passing facilities is
presented. Space requirements for safe, lawful, and comfortable same-lane
passing between cycle traffic and a variety of larger vehicles under different
conditions is discussed. Effective positioning of cyclists according to best
cycling practices is used as a reference point around which overtaking
facilities may be designed. Understanding of effective bicycle driving
techniques allows roadway engineering to be done in a manner compatible with
safe and lawful cycling.

Introduction

Ordinary (non-freeway) streets carry a diversity of vehicle types at a wide
range of speeds. State law assigns operators of all of these vehicles the rights
and responsibilities of drivers on roadways. In order to reduce potential social
friction and user stress among drivers who wish to travel at different speeds,
traffic engineers often design roadways to facilitate overtaking. When a
driver traveling in a marked lane overtakes a narrow vehicle (such as a bicycle),
the width of the lane is a critical factor in the driver's decision to pass by
either (a) changing lanes, or (b) staying within the same lane as the narrow vehicle. Changing lanes
to pass requires a driver to find an appropriate gap in traffic in the adjacent
lane. Under most conditions this can be done easily and conveniently. But if
there is substantial traffic in the adjacent lane, or if sight lines are poor,
the driver may have to wait some time for an appropriate passing opportunity.
The driver must meanwhile travel at the speed of the slower traffic.

The overtaking of bicycle traffic by automobile traffic has gained particular
interest among the public and the traffic engineering community because of the
potential difference in speeds and the popularity of both modes for travel on
important roads that serve important destinations. Some motorists may feel
inconvenienced by the presence of bicycle traffic, and some cyclists may feel
intimidated by situations where they believe they may be creating inconvenience
for automobile users. In traffic situations where changing lanes to pass
cyclists may be awkward or inconvenient, traffic engineers may facilitate
passing without changing lanes by providing adequate pavement space for such
overtaking to occur at safe and legal distance. Drivers wishing to overtake
slower users are relieved of the legal and necessary burden of yielding to the
adjacent line of traffic when that traffic is positioned far enough to the
left. This can be facilitated through design by moving the left-hand lane
stripe far enough to the left, creating a wider lane. An example of such a wide
lane is shown in Figure 1.

Figure 1: This wide lane allows automobile drivers to easily
overtake cyclists without changing lanes, and is comfortable for casual cyclists
to use even cycling two-abreast. (Wayne Pein Photos)

Wide lanes are sometimes referred to as bicycle facilities in
traffic engineering literature. But strictly speaking, wide lanes are not
required for bicycle travel, which is allowed on all ordinary roads; the extra
width provides an optional improved passing facility. Roads with narrow lanes
often carry substantial bicycle traffic; drivers of wide vehicles cannot pass
these cyclists at safe and lawful distance
without moving into the next lane, as shown in Figure 2. This is the typical practice
seen on narrow-lane roads. But cyclists occasionally experience drivers attempting to
pass at unlawfully close and unsafe distance within the same narrow lane,
particularly when there is traffic occupying the adjacent lane. Although unsafe
passing is primarily a law enforcement issue, this behavior is less common in
wide lanes. As a result, many cyclists as well as motorists are proponents of wider pavement on roads that carry substantial traffic as a way to
minimize the potential for conflict.

Figure 2: This 10' wide lane does not provide enough room for
automobile drivers to overtake cyclists at safe and lawful distance within the
same lane. Automobile drivers routinely move into the adjacent lane to pass. (Wayne Pein
Photo)

Wide lanes work equally well for motorists overtaking other low-speed narrow
vehicles such as mopeds and motor scooters, and depending upon width, facilitate
overtaking of stopped buses, delivery trucks, and slowed right turning
vehicles. Bicyclists also enjoy such passing advantages when they are the faster
vehicle.

As the number of automobile-cyclist overtaking events increases, the
community benefit of enhanced passing facilities increases. But there are a
large number of factors that may affect the decision to outfit a road
with such a space arrangement. These may include bicycle and motor vehicle
traffic volumes, design speed, cyclist demographics, available right of way, cost of construction, and
pedestrian crossing distances. Communities will likely weigh these issues on a
case-by case basis, and may choose to concentrate on education and enforcement
to promote safer behavior by overtaking motorists in those areas where roadway improvements are not made. A
discussion of the effects of bicycle traffic on trip time and roadway capacity
is provided by Forester [1]. Warrants or guidelines for when
to provide passing facility enhancements are outside the scope of this article.
Also, since different streets will carry different types of traffic at different
concentrations and speeds through different types of neighborhoods, there is no
perfect, one-size-fits all overtaking facility design that is always most
appropriate. This article concentrates on the functional design issues related to best
practices of bicycle operation and safe, lawful overtaking so that road
designers can apply these basic principles to different streets in context.

How Much Width?

The total width required to afford safe, lawful, and comfortable overtaking
of cyclists by wider vehicles includes the width of the vehicles, users' shy
distance to each side of the travel lane or adjacent traffic, and adequate
spacing between them. The relationships between these distances are illustrated in Figure 3.

Figure 3: Distribution of space required to accommodate parallel operation of
a bicycle and a typical motor vehicle (e.g. an SUV).

The component distances are as follows:

A. Shy distance on the paved surface between the cyclist's wheels and
the gutter pan seam, drain grate, curb, or other surface discontinuity,
whichever is closest. This space is required for maneuvering, balance and recovery. Typically between two and four feet, this may be
even wider when descending a hill at high speed, when sight lines
are poor, or where edge surface conditions are poor, and may be narrower at
slower speeds or where the gutter pan is smooth and flush with the travel lane.
On non-curb-and-gutter roadways, the cyclist's tires should also be assumed to
be to the left of the fog line in order to avoid debris on the shoulder or to
avoid a right-turn-only lane if present.

B. One-half of the width of the cyclist (typically just over
one foot) or, in the case of wider vehicles such as trikes and trailers, the
distance of the left edge of the vehicle to the rightmost wheel.

C. Passing distance between cyclist and automobile - three feet
is the legal minimum in those states that define it specifically for overtaking
of cyclists; most DOTs and state-endorsed safety materials recommend this as an
absolute minimum. This minimum space is recommended because cyclists are exposed
and thus more vulnerable on open vehicles, because they require some wiggle room
for balance and recovery, and because wind blasts from passing vehicles can destabilize cyclists. Drivers are always required by law to operate safely, and as
vehicle speeds increase, more that three feet of passing space is required for
safety. At slow speeds, some cyclists feel comfortable with less than three feet
of clearance space provided that they have three feet or more of recovery
area to their right. Empirical evidence on real-world streets typically
yields average passing distances of four to five feet, and substantially more
where traffic and roadway space allow.

D. Width of the motor vehicle including mirrors, typically
just over 6 feet for SUVs, and well over 10 feet for some buses, commercial trucks and
RVs.

E. Shy distance between the motor vehicle and the adjacent
traffic lane. Comfortable distance is usually at least one foot.

As shown in Figure 3, when a cyclist operating with his tires
2.5 feet from the gutter seam is passed by an SUV at a typical distance of about four
feet, the left side of the SUV will be about 14 feet from the gutter. With
12-foot lanes, the driver encroaches two feet into the adjacent lane.
(Real-world road cyclists observe that such encroachment is fairly typical on
roads with 12' lanes) A 14-foot wide lane accommodates this overtaking
maneuver with no shy distance to the adjacent lane, but with little or no
encroachment. A 16-foot wide lane provides about two feet of shy distance to the
left edge of the lane. This shy distance is most desirable where there is
oncoming traffic in the adjacent lane or if the left edge of the lane is
bordered by a raised center median.

Any increase in lane width above 12 feet can provide some
improvement in passing convenience and comfort under many conditions, even if it
is just reducing the amount of encroachment by overtaking drivers into the
adjacent lane. The more width is provided, the greater the number of different
vehicle types (such as vans and wide trailers) that can pass at safe distance
without crossing the lane line, and the greater the buffer space provided by
drivers of normal-width cars. A width of 13 feet is better for overtaking than
12; 14 is better than 13; 15 is better than 14. The North
Carolina Department of Transportation publication North Carolina Bicycle
Facilities Planning and Design Guidelines makes the following
recommendations:

On roadways that accommodate [passing between] both
bicycles and motor vehicles within the travel lanes, 4.2 m (14 feet) of usable
width should be provided on the outside through lanes. Studies have shown that
any additional width on outside through lanes is beneficial. In determining
the usable width of an outside through lane, adjustments need to be made for
obstructions. Bicyclists shy away from obstructions such as drainage grates,
parked vehicles and longitudinal ridges between the pavement and gutter
sections. An extra 0.3 m (1 ft) of "shy distance" should be added
for flush or depressed obstructions, such as a joint or soft shoulder.

Many municipalities use standard lane widths of 16 feet for
two-lane roads because this width allows cars and emergency vehicles to pass
parked or disabled vehicles without crossing the center line or mounting a
center median if present. A 16-foot lane also provides just barely enough space
(with some overhang into the next lane) for a city
bus or truck to pass a cyclist safely when done at slow speed, as shown in
Figure 5. Figure 6 depicts an SUV driver overtaking to the right of a cyclist
preparing to turn left from near the center of the roadway, as the cyclist is
required to do by state law. Left-turning cyclists prefer to have some shy
distance between their left hand signal and oncoming traffic. A 16-foot wide
lane provides this space plus adequate space for safe and lawful overtaking by
the SUV driver. Figure 7 depicts a cyclist safely and lawfully overtaking a
stopped city transit bus.

Figure 5: Minimum space required for low-speed overtaking by a
city bus with
a mirror span of 10.8 feet. Greater passing distance is required if the bus is
traveling at high speed.

Figure 6: SUV overtaking to the right of a left-turning
cyclist.

Figure 7: Cyclist overtaking to the left of a stopped city
transit bus.

When large vehicles such as trucks and buses overtake a cyclist
at high speed, the wind blast can destabilize the cyclist and cause him or her
to swerve in a hazardous manner. This wind blast is a function of distance and
speed, as shown in the chart in Figure 8. This chart appears in North Carolina Bicycle
Facilities Planning and Design Guidelines. For higher-speed arterial roads
carrying truck traffic at speeds of 50 mph and above, the
minimum distance for a tolerable wind blast force ranges from four to six feet.
But "tolerable" is likely a stronger wind blast than what would be
considered "comfortable" by some cyclists. Roadway designers may wish
to consider providing greater passing distance where high speeds and/or volumes
of truck traffic are expected. Education and enforcement of vehicle
operators to reduce their speed when passing is another strategy that can
improve cyclist safety and comfort.

Figure 8: Wind blast forces on cyclists overtaken by heavy
motor vehicles as a function of distance and speed.

The North Carolina Bicycle Facilities Planning and Design
Guidelines suggests that 16 feet is the desired minimum amount of total
space for safe and comfortable passing of cyclists. In the section on bicycle
lane widths, the Guidelines states:

Under ideal conditions, minimum bicycle lane width is 1.2 m
(4 ft). However, certain edge conditions dictate additional bicycle lane
width. Additional width is also desirable if the width of the adjacent traffic
lane is less than 3.6 m (12 ft). This is an important addition because the
effective clearance between bicyclist and the adjacent traffic is a function
of the combined width of both the bike lane and the adjacent traffic lane.

From the preceding analysis, it should be obvious why many cyclists report greater comfort
operating where 16 feet of total pavement width is provided for side-by-side
operation (bike lane stripe or not) compared to narrower roads with the same
traffic volume. Note that bike lane striping can be described accurately as the
segregation of traffic by vehicle type within a through lane that is 16' wide or
wider. Segregation by vehicle type has numerous disadvantages, especially where
it conflicts with proper destination positioning (such as in Figure 6) or speed
positioning (as in Figure 7), and has
never been shown by scientific study to improve safety for cyclists. For deeper
analyses comparing plain wide lanes to segregated lanes, see Pein [2],
[3], and Goodridge [4].
For additional discussion of optimum wide outside lane widths, see Pein [5].

Placement of Wide Outside Through Lanes

There has been very little discussion in the highway engineering
literature of how to effectively configure wide outside lanes at intersections.
This is an important topic, because ineffective allocation of roadway space at
intersections may waste the opportunity for easier overtaking or discourage safe
and lawful positioning of cyclists traveling through the intersection. When
provided in the context of bicycle transportation, a wide outside lane should
normally be a through lane, not a right-turn-only lane. This is because
if most of the traffic is headed straight through the intersection, and cyclists
operate lawfully in the correct lane for their destination, then most of the
same-lane automobile-overtaking-bicycle events will occur in the rightmost
through lane. Traffic in the right-turn-only lane will be lower in volume, as
well as comparably slower in preparation for the turn. The North
Carolina Bicycle Facilities Planning and Design Guidelines uses the term
"through lane" extensively in its discussion of wide outside lanes;
for instance, from page 27: "At intersections with separate
right-turn-lanes, the outside through lane should be widened...."

An example of a wide outside through lane beside a
right-turn-only lane is shown in Figure 9. If the through lane turns into a
right-turn-only lane, cyclists must merge left into the through lane. To provide
the same overtaking benefits for through-traveling automobile drivers, the space
in the outside lane must be redistributed to the through lane. This can be
accomplished with a taper, as shown in Figure 10. The distance over which this
transition takes effect should be on the order of a hundred feet or more
in order to allow cyclists time to negotiate a merge prior to reaching the
intersection.

Long, high-speed right-turn-only lanes can make cyclists
uncomfortable where they result in traffic overtaking cyclists on both sides at
high speeds. This becomes especially uncomfortable for cyclists if the through
lane is narrow, thus increasing their friction with through traffic. Keeping the
through-lane substantially wider than the right-turn-only lane encourages
through-cyclists to stay in the correct lane. Minimizing the use of high-speed
right-turn-only lanes in general may also improve cyclist comfort and reduce the
problem of improper cycling.

On-Street Parallel Parking

On-street parallel parking requires cyclists to
allow themselves additional shy distance away from the door zone. The sudden
opening of car doors directly in front of cyclists is a common
cause of bicyclist crashes and injuries in urban areas (see survey of research
by Allen [6]). Cyclists are more narrow
than approaching automobiles and are therefore more likely to go undetected by
passengers about to disembark from parked cars. A cyclist operating at speed
will often be unable to stop or merge to the left safely in time when a door is
opened without warning. For these reasons, traffic cycling training provided by
organizations such as CAN-Bike and the League of American Bicyclists teaches
cyclists to ride at least five feet away from the side of
parked cars to eliminate the risk of dooring.

Some traffic engineering departments have
attempted to facilitate convenient automobile-overtaking of cyclists by
directing cyclists to ride within the door zone by marking bike lane stripes and
stencils in the door zone, as shown in Figure 11. Experts in cycling safety
charge that this is a highly unethical practice that directly contradicts
safe-cycling instruction and best bicycling practices. Cyclists operating in
door-zone bike lanes have been injured and killed (see Allen,
[7]) as the result of the sudden opening of doors.

If space for overtaking of cyclists operating in a safe manner
is to be constructed in the vicinity of on-street parking, the cyclists' shy
distance must be increased to keep them well outside of the door zone. A
discussion of door zone width and a table of widths for different vehicles is provided by
John Allen in [8]. Figure 12 shows the
spacing of an SUV overtaking a cyclist tracking a safe distance (B+A+DZ) away from
a parked SUV of width W located at a distance F from the edge of the parking
space. The width required for a city bus to make the same
maneuver is shown in Figure 13. If this space cannot be provided with a wide
enough lane, then cyclists should be encouraged through an education campaign to
operate near the center of the travel lane, and motorists should be encouraged
and enforced to change lanes to pass cyclists.

Engineering impacts on the safety, comfort, and convenience of
motorist-overtaking-cyclist events are primarily a function of the provided
pavement space. Passing-facility enhancements can make cycling more pleasant,
but they are not bicycle facilities; the cycling facilities are, by state law,
already there. Travel lanes narrower than 12 feet do not allow drivers of cars
and wider vehicles to pass cyclists entirely within the same lane at safe and legal
distance. Lanes substantially wider than 12 feet, e.g. 14 to 16 feet, allow for
more possibilities for same-lane passing among a wider variety of vehicle types,
with greater safety and comfort for both cyclists and overtaking drivers. Where
right-turn-only lanes are provided at junctions, lane width enhancements related
to overtaking of cyclists should be placed in the through lane where
through-cyclists will operate. Near on-street parking, design of overtaking
facilities must accommodate the need for cyclists to operate well outside the
door zone. Since overtaking facility improvements are not feasible or advisable
everywhere, public education and enforcement are important to remind overtaking
operators of their duty to respect the safety and travel rights of slower
operators in travel lanes.